Continuous ink-jet printers have been employed in industrial use for many years for marking a wide variety of products. The working principle for these ink-jet printers in the past has functioned such that an ink to be applied is supplied from a reservoir via pumps with positive pressure to a pressure chamber that is provided in the actual print head and that has a nozzle on its side facing the article to be printed.
The nozzle has an opening diameter in the range of e.g. 30 μm to 200 μm. The ink jet exits from the nozzle initially as a continuous ink jet, but this is not useful for printing because the characters produced in this type of printing job are constructed from individual points or individual ink drops.
In order to break down the ink jet into uniform individual ink drops, attached to the pressure chamber is a modulation element that creates pressure fluctuations in the exiting ink jet so that a short time after it exits from the nozzle the ink jet breaks up into uniform individual ink drops at a defined spacing. The size of the ink drops is a function of the modulation frequency applied, nozzle diameter, and the pressure produced by the pump, and it can be adjusted within the limits for the system that are prescribed by the combination of the above-described parameters. It is not possible to vary the drop size of successive ink drops.
Shortly before the ink drops are formed from the ink jet that has exited, the ink drops are each individually provided with an electrical charge, the amount of the charge being a function of the desired impact position on the product to be marked. The ink is slightly electrically conductive in order to ensure the electrical charging.
During the charging process, the ink drop has not yet been broken off from the ink jet that has exited from the nozzle of the ink-jet printer so that due to the electrical influence free charge carriers in the ink are moved toward or away from the charge electrode, depending on the polarity and strength of an external charge voltage, the ink chamber and thus the ink reservoir being held for instance electrically to ground potential. The charge electrode has no mechanical contact with the ink jet.
If the ink drop now breaks off from the ink jet while it is in the field region of the charge electrode, the electrical charges that have migrated into the drop due to the influence remain in the drop volume and the latter is also electrically charged to the outside even after it has broken off. If the charge electrode is positively charged, for instance, when the ink jet enters the electrical field of the charge electrode the negative free charge carriers in the ink migrate into the field, while the positively charged free charge carriers in the ink are displaced from the electrical field.
Thus a charge separation occurs on the front edge of the ink jet immediately before the drop breaks off and the charge equilibrium thus produced is maintained in the drop that is breaking off and the drop leaves the field region of the charge electrode, in this example with a negative charge.
Since, due to structure and principle, the ink drop breaks off during the period in which the charge voltage influences the drop, as described, a charge remains on the ink drop that has separated and the amount of the charge corresponds to the amount of the applied charge voltage given constant electrical conductivity of the ink and thus, 0given a change in the charge voltage, the charge in each drop can also be altered.
During their travel, which is initially in a straight line, the electrically charged ink drops successively enter the electrostatic field of a plate capacitor and are more or less deflected from their straight trajectory depending on their individual charge and after leaving the electrostatic field continue their travel at a specified angle to their original trajectory, the angle being determined by their charge.
With this principle it is possible to select different impact positions on a surface to be printed with individual ink drops, this occurring only in one deflection direction in this embodiment. For blocking individual drops from the image zone or if printing is not to occur, the ink drops are given a certain fixed charge or remain uncharged so that after they exit from the electrostatic field of the plate capacitor they are captured in a collection tube, from which location they are pumped back to the ink tank via a pump system. Thus the ink that is not used in printing is circulated in the cycle, which is why it is called a continuous ink-jet printer.
In the above-described conventional embodiment it is disadvantageous that, due to the system-imposed manner of deflecting the ink drops, the ink itself must be electrically conductive, even if only slightly, so that the individual charge required for the electrostatic deflection can be applied to each individual ink drop.
This limits the number of inks that can be employed, since it is not possible or useful to provide for each desired ink composition electrical conductivity itself or via additives. For instance, there may be an ink that has magnetic properties. Such an ink could be created to be electrically conductive, for instance by means of an additive, but then it would not be possible to control the trajectories of the individual ink drops based on the induction that occurs and the associated different additional deflection forces.
In contrast to this, DE 103 07 055 describes a method for deflecting ink drops that, by means of an ultrasound wave, depending on expended sonic energy, deflects with different strengths the ink drops produced in the normal manner by pressure modulations in the ink.
It is advantageous in terms of this type of deflection that the inks to be printed do not have to be electrically conductive, which makes it possible to use a great number of very different inks with different properties.
For the system described in DE 103 07 055, it is disadvantageous that, first, drop production and drop deflection must be precisely synchronized, which also must take into account the final propagation speed of the sound waves at the site of the deflection as a function of the locally prevailing ambient conditions in order to make it possible to precisely deflect an ink drop to the desired position. It is furthermore disadvantageous that when using a simple sound generator, due to the size of the sound-generating surface, the acting acoustic energy acts not only exclusively on the ink drop to be deflected, but rather at least in part also on preceding and subsequent ink drops, so that precise deflection of the ink drops is only possible under certain conditions. It is furthermore disadvantageous that, due to their generation, the deflected ink drops are all the same size, so that type faces with different line widths cannot be produced without overlaying a plurality of ink drops and thus can only be produced in steps.